The machine tool may be a machine for manufacturing or machining gearwheels.
Such a device frequently is referred to as face driver or end driver and may be used in undulating workpieces which have no or an insufficient workpiece bore from which the workpiece can be clamped for machining. Furthermore, such devices are used when an outer surface of the workpieces shall be machined along their entire length in one clamping position. Such devices are known from a plurality of applications, for example from DE 195 34 073 A1 or DE 10 2010 060 118 A1.
For machining the workpieces, the workpieces initially are clamped between tips, in order to ensure a workpiece concentricity, in compliance with the workpiece center. At the same time driver elements, which for applying a high torque are arranged with a rather large distance concentrically to a tip (e.g., centering tip), are pressed against a drive-side end face of the workpiece for rotary entrainment. Depending on a surface hardness of the workpiece and depending on the end-face shape and size of the driver elements, the driver elements are pressed into the surface of the workpiece to varying degrees.
A shape and size of the impression surface may be chosen such that a secure rotary entrainment of the workpiece to be machined is ensured with a chosen machining method. Furthermore, care may be taken that the workpiece is guided by the tip and not lifted off from the same by a pressing force of the driver elements, so that a precise workpiece concentricity remains ensured.
As at a time of machining the end faces of the workpieces frequently do not yet have a sufficient axial runout, but nevertheless all driver elements may uniformly be brought in engagement with the workpiece surface, the driver elements may contact the workpiece surface independent of each other and then nevertheless apply a rather uniform pressing force. In DE 20 2010 009 973 U, the pressing force for example is uniformly applied hydraulically on all driver elements. In other embodiments, driver elements resting on an oscillating ring or a gimbal-mounted disk compensate errors. In some embodiments the driver element also includes an oscillating mounted disk with a plurality of driver cutting edges.
The centering tip in the device either is mounted in an axially compliant fashion or designed stationary in a basic clamping position. As compared to an axially compliant tip, a firm assembly offers an advantage of workpiece concentricity, as bore clearance may be reduced between the tip and the basic clamping position, in order to ensure an axial shiftability (e.g., movability) thereof.
In addition, a position of the driver elements may not change during machining. In workpiece machinings in which forces with alternating directions of rotation act on the workpiece or in a case of greatly fluctuating machining forces, (for example, with interrupted cuts), the driver elements can move in their bore under an alternating load. In the case of long shafts a workpiece bend, due to the unilateral machining engagement, can lead to driver elements being loaded differently during the workpiece rotation and axially shifted thereby.
In workpiece machinings in which an exact rotary position is desired for a machining result, e.g. during tooth-machining, each change in the rotary position between device and workpiece has a negative influence on a generating coupling between tool and workpiece. Here, a very precise coupling between the rotary position of the workpiece and the rotary position of the tool increases toothing quality.
One possibility to increase toothing quality is a guidance of driver elements (which may herein be referred to as driver pins) in their bores. Care may be taken for a sufficient bore clearance, so that the axial movability of the driver pins is maintained, but at the same time the clearance may not become too large, so that a precise entrainment is achieved.
In case of very high quality requirements, this was found to be not sufficient. In DE 20 2010 009 973 U, axes of the driver elements therefore were inclined against a direction of rotation of the workpiece and hence no longer extend parallel to a workpiece axis. The workpiece in engagement with the driver pins thus generates a force directed opposite to the direction of rotation, which together with the force acting axially on the driver pins leads to the fact that in the clamped condition the driver pins are canted in the bore without clearance and thus increase rotary entrainment. Depending on the design, a partial quantity of the driver pin each can be inclined in and against the direction of rotation of the workpiece.
Nevertheless, alternating loads of an interrupted cut or machining forces acting in different directions of rotation can lead to the driver elements not remaining braced in the clamped condition. Due to alternating forces, canting can be released temporarily and thus allow small movements in radial direction and hence lead to a change in position of the workpiece with respect to the device. This is reflected in the machining result, for example as a shoulder on a tooth flank or as tooth direction error.
These loads can occur for example during gear hobbing or during gear grinding. This effect occurs on a cutting lead and on exit of the tool from toothing, when the tool cutting edges are in engagement on one flank. When helical toothings are machined, this effect becomes apparent even more distinctly due to mutual contact of the tool with left and right tooth flanks.
It therefore is the object of the present disclosure to address the issues mentioned above such that a rotary entrainment without clearance is achievable in all machining methods. In addition, secure clamping may not change during a machining process, not even when tangential machining forces fluctuate in terms of magnitude and direction.
This object is solved by a machine tool device configured to receive a workpiece between two tips, the machine tool device comprising: a plurality of driver elements arranged concentrically around a central axis of a lower tip for rotary entrainment of the workpiece, wherein each driver element of the plurality of driver elements is adjustably mounted in an axial direction of the workpiece in a lower part of the machine tool device; wherein each driver element is configured to engage with an end face of the workpiece; and wherein the plurality of driver elements is coupled to an actuator for selective engagement of the driver elements with the end face.
In the device (which may herein be referred to as a machine tool device) with face drivers according to the present disclosure, the workpiece to be clamped initially is centered on a fixed tip in the device and on an opposed tip. As compared to a movable tip, the fixed tip offers the advantage that it can be aligned very precisely on the machine table center and no bore clearance, which would be necessary for a movable tip, disadvantageously influences the workpiece concentricity. After the centering operating, the driver elements are applied to the workpiece end face.
Constructively, the driver elements for workpiece entrainment may be arranged radially with a rather large distance to the tip, so that a rather high torque can be transmitted from the device to the workpiece. Depending on the workpiece shape, individual driver elements also can be arranged with a different distance to the center. At the same time, there is a requirement for a rather lean device so as to prevent a collision of a machining head or tool with the device, especially when manufacturing helical toothings.
To compensate axial runout errors of the workpiece end face, the driver elements initially may compliantly rest against the surface and thereafter be pressed on or into the surface with approximately equal pressure. It thereby is avoided that the workpiece is tilted due to non-uniform or unilateral pressure on its end face and thus wobbles relative to the device.
In the device according to the present disclosure the driver elements therefore initially are retained mechanically when the workpiece is attached to the tip. After a precise alignment of the workpiece between the tips and an application of the centering force, which prevents shifting of the workpiece on the tip, the driver elements are released by actuating a tension bolt (e.g., drawbar), in axial direction of the device, and thus can rest against the workpiece end face or press into the workpiece with an adjustable force. When the tension bolt in addition is actuated further in axial direction, the driver elements are actively clamped in their end position via additional clamping elements and thus fixed in their position without clearance during machining of the workpiece.
This clamping process on one hand ensures a good workpiece concentricity and the workpiece is also securely guided between tips with alternating machining forces. The torque for machining is applied and maintained and the precise rotary transmission is ensured.
In a further embodiment it is conceivable that the upper tip is formed as hollow tip or steady rest. Alternatively or in addition a design of the lower tip as hollow tip is conceivable. The guiding element ensures a concentricity of the workpiece.
In an alternative embodiment, the tip in the basic clamping position can be mounted compliant in axial direction. By adjusting spring force and axial paths between the tip and the driver elements, a similar clamping process can be achieved. Springs and axial paths may be adjusted to each other such that the centering operation first is completed, before the driver pins are brought in engagement with the workpiece end face. In this embodiment, the bore for guiding the tip has a small clearance. Therefore, this embodiment chiefly is used when the workpiece shape does not provide the use of the embodiment.
In another embodiment, the contact surfaces of the driver elements additionally are equipped with replaceable attachments. Thus, adaptations to the material properties of the workpiece to be machined, the shape and size of the admissible impressions at the workpiece, the treatment condition of the workpiece material, but also to the machining method can be carried out quite easily. For example, a supporting surface for a gear milling process and a support for a hard finishing process may be designed differently for a single workpiece. On the one hand, processes differ by the occurring machining forces, and on the other hand a hardened edge layer constitutes an obstacle against a penetration of supporting points. In this case, a frictional entrainment by the driver elements is desirable.
Furthermore, the driver elements can be provided with different attachments on upper end faces. The device thereby can be adapted quite easily to different workpieces, and in a case of wear attachments can be exchanged without problems. It can be advantageous to fall back on commercially available attachments, such as indexable inserts.
Due to possibility for adjustment of the spring force of the driver elements, the force can correspondingly be adjusted in compliance with properties of contact surfaces (frictional contact, penetration depth, etc.). By means of a variable tensile force, height of the active clamping force for the driver elements can be influenced in addition. At a reduced clamping force, instead of an active fixation of the driver pins, the driver pins could also work with a damping effect.
It can furthermore be provided that the upper and/or lower tip for receiving the workpiece likewise can be designed as change part, in order to adapt the tips to differently large workpiece bores and workpiece dimensions and to likewise exchange the tips easily and at low cost in a case of wear.
An extended embodiment can be used for workpiece loading in case of manual loading, without use of an automation equipment.
Manual loading requires a centric attachment of the workpiece to the device, in order to prevent tilting of the workpiece. Possible falling of the workpiece during loading and a possible hazard to the machine operator thereby shall be prevented. Loading of machines for example with long shafts frequently is realized in a hanging manner with a loading device, optionally an eye bolt which is screwed into an upper central bore of the workpiece. Such a device subsequently may be removed, however, before the upper tip or the upper guiding element can move onto the upper side of the workpiece. During manual loading, the device therefore may include a suitable means for holding the workpiece.
During automatic loading, the loading device prevents tilting of the workpiece.
In an extended embodiment, tilt protection is realized via coaxially arranged supporting bolts/receiving elements, on which the workpiece can be placed during loading. The same however are retracted from the workpiece end face for subsequent machining of the workpiece, in order to prevent a negative influence on quality of concentricity/axial runout of the workpiece.
Further advantages and properties of the present disclosure will be explained in detail below with reference to several drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.